1. Field of the Invention
The invention relates to an annular ferromagnetic yoke for deflection units for television display tubes, having a conical inner surface about a longitudinal axis, and having an outer surface in which a first and a second set of grooves extending in the direction of the longitudinal axis are provided symmetrically with respect to a first plane through the longitudinal axis.
2. Description of the Prior Art
Such an annular yoke is known from the article "Large screen colour television with intrinsically convergent 110.degree. deflection" by P. G. J. Barten and J. Kaashoek published in Electronic Applications Bulletin 33, No. 2, pp. 75-88. FIG. 12 of this article shows an annular yoke which consists of two combined yoke halves which generally are obtained by breaking or cracking into two parts a sintered yoke formed as a single ring. Their halves are held together by means of two clamping springs engaging in abutting surfaces near the contact surfaces of the yoke halves extending in the direction of the longitudinal axis. These abutting surfaces are formed by the side walls of the grooves which are provided in the yoke ring during pressing, or by the side walls of ridges or wings which are pressed on the yoke ring. As is known, however, a ring is deformed during sintering so that the inner surfaces of sintered yoke rings generally are not truly rotationally symmetrical, while in addition the diameter of the yoke rings mutually varies. The presence of pressed grooves, cracking seams and winding projections and pressed ridges, respectively, leads to a deterioration of these conditions as a result of the associated variation in the wall thickness of a yoke ring. The above makes it necessary to accurately finish the inside of the sintered yoke rings. This finishing treatment (grinding) is the more complicated the larger the deformation of a sintered yoke ring. However, this is not the end of it. During assembly, the internally truly rotationally symmetrically ground ring must be centred around the remainder of the deflection unit via the outer surface which is no longer concentric with the inner surface. The advantage of grinding the inner surface is then partly lost.
Of late years the development of picture display tubes has been to a considerable extent towards constructing display tubes having ever smaller dimensions in the axial direction. For that purpose the electron beam or beams generated in an electrode system is or are deflected over an ever larger angle. Starting with a deflection angle of 70.degree. said development has continued via a deflection angle of 90.degree. to the nowadays usual construction having deflection angles from approximately 90.degree. to approximately 110.degree.. The problems associated with this development not only relate to the construction of the ferromagnetic yoke ring but also to the positioning of toroidally wound deflection coils thereon and the alignment of the yoke ring (on which coils are wound) relative to the remainder of the deflection unit. As the deflection angle increases, more stringent requirements are imposed upon the accuracy with which the yoke ring can be positioned not only in an axial direction but in particular also in a radial direction. Particularly in deflection units for combination with colour television display tubes it is important that the positioning of the deflection coils on the yoke ring and the centring of the yoke ring should occur as accurately as possible so as to minimize colour defects as a result of mislanding of an electron beam on the display screen.
It is the object of the invention to provide an annular ferromagnetic yoke of the kind mentioned in the opening paragraph which presents the possibility of accurate alignment during assembly, and/or presents the possibility of positioning deflection coils thereon accurately.